1. Field of the Invention
The present invention relates generally to network computing, communications protocols and, in particular, to fiber optics.
2. Description of Related Art
In recent years, the Fibre Channel Standard has emerged as a preferred protocol for data communication in enterprise storage area networks, disaster recovery applications, and other applications. Changes were made to the layer 4 protocols of Fibre Channel, resulting in the Fibre Connection (FICON) protocol for enterprise mainframe computers. Both of these protocols are currently used in servers and storage at data rates of 1 and 2 Gbit/s, with extensions to 4 Gbit/s and 8-10 Gbit/s in the near future. These protocols are often extended over distances in the 50-100 km range, and future applications are expected to further extend supported distances to several thousand km.
This channel extension is currently accomplished by using fiber optic wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) equipment. Currently, channel extensions are provided over dark fiber, which is dedicated to a specific customer; in the near future, it will be provided by encapsulating the data into synchronous optical network (SONET) frames using standards such as Generic Framing Procedure (GFP). In either case, there is a concern with the potential for duplication or dropped frames on extended distance links. This concern applies to other protocols as well, including the InterSystem Channel (ISC) links used in a Parallel Sysplex, ESCON channels, and similar protocols. The ISC links that carry System Timer Protocol (STP) information also need to prevent duplicated or lost frames over extended distances.
FIG. 1 shows a Fibre Channel extended buffer feature in the prior art. There are links 102 (a/k/a fibers or paths) between two DWDMs 100. Typically, these links 102 are configured with a working path (a/k/a primary path) and a backup path (a/k/a secondary path or protection path) over a physically diverse route so that a break in the working path automatically causes a failover switch to the backup path. Typically, the same data is sent down both paths and selected by a protection switch at the receiving end of the link 102. In general, the two fibers are not of equal length. When the backup path is significantly longer than the working path, a significant amount of data may be in transmission over the longer fiber link, corresponding to a latency of about five microseconds per km, for example. If a fiber breaks on the working path, data frames that are still being transmitted over the backup path may arrive as duplicates at the receive end of the link. Similarly, some data frames may be lost during a protection switchover, which typically is about 50 ms long. Duplicate or lost frames are a significant problem in the Fibre Channel protocol; in the case of long links, entire sequences or exchanges may be duplicated, further complicating the problem.